Support C++ custom algorithms

[siyuan0322]

We could let user write a python algorithm and load it to GraphScope and run query with it.
However, it's pretty naive compared to C++ algorithms, we need to expose the entrypoint to plug in a UDF C++ algorithm.

[wuyueandrew]

im looking forward to c++ udf.

[sighingnow]

We do support user-defined algorithms in C++, by using a gar to initilaizing an AppAsset.

[siyuan0322]

Yeah，the essential components are ready but we do need to try this whole process from the beginning to the end and present it formally.

[sighingnow]

See https://github.com/GraphScope/cpp-template

[siyuan0322]

See https://github.com/GraphScope/cpp-template

Nice template! I'll try it myself. And I need to bootstrap a pregel example.